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Hyperkalemic periodic paralysis

Hyperkalemic periodic paralysis (HYPP), also
known as Impressive Syndrome, is an inherited autosomal dominant
disorder which affects sodium channels in muscle cells and the
ability to regulate potassium levels in the blood of horses.
This inherited disease is characterized by uncontrollable muscle
twitching and substantial muscle weakness or paralysis among
affected horses. HYPP is a dominant disorder; therefore heterozygotes
bred to genotypically normal horses will still likely produce
clinically affected offspring 50% of the time.

The disease is contained to the bloodline of
the famous Appendix American Quarter Horse stallion Impressive,
who has over 55,000 living descendants as of 2003. Although
the disease is primarily limited to the American Quarter Horse
breed and closely related breeds such as American Paint Horses
and Appaloosas at this time, cross-breeding has begun to extend
it to grade horses and ponies. The spread of the disease is
perpetuated by the favorable judgings given to diseased horses
in showing, due in part to involuntary muscle twitching which
helps to build large, bulky muscles that judges favor.

In 1994, researchers at the University of Pittsburgh,
with a grant from various horse organizations, isolated the
gene responsible for the problem and developed a blood test
for it. Using this test, horses may be identified as:

H/H, meaning they have the gene and it
is homozygous. These horses always pass on the disease.

N/H, meaning they have the gene and it
is heterozygous. These horses are affected to a lesser degree,
and pass on the disease 50% of the time.

N/N, meaning they do not have the disease
and cannot pass it on, even if they are a descendant of
Impressive.

Recently, horse organizations have begun instituting
rules to attempt to eliminate this widespread disease. The American
Quarter Horse Association (AQHA) now mandates testing for the
disease and will no longer register homozygous (H/H) foals as
of 2007, with discussion of heterozygous (N/H) foals pending.
The Appaloosa Association will no longer accept homozygous foals
as of 2008. It is believed that both primary palomino registries
will exclude any HYPP-carrying foal as of 2007. The main organization
affected by HYPP that has not yet taken action is the American
Paint Horse Association (APHA), although many other smaller
organizations are also affected.

Although much rarer, hyperkalemic periodic
paralysis has also been observed in humans. The most common
underlying cause is one of several possible point mutations
in the genes synthesising calcium or sodium ion channels in
skeletal muscle; this mutation can either be inherited in an
autosomally dominant or recessive manner.